This whole lash up reminded me of that tactical nuke Mortar the Army developed back in the 50's.
I assume your thinking of the M-28 / M-29 'Davey Crockett' nuclear recoilless gun. It only had a variable yield of 10 or 20 tonnes (yes, 0.020 kT), though the radiation was at instant lethal levels at ranges of over 400 meters, the weapon itself having a range 2000 meters. Needless to say if you fired it with the wrong winds or at too close a target the weapon effects would kill the operators, but in correct operation the weapons 17 second flight time gave the firing party enough time to dive into a slit trench to avoid the majority of the weapon effects. The stated fact that the weapon instant effects exceeded the weapon's maximum range is a load of BS. This site has a good powerpoint presentation link on it showing the attack profile of a WE-177 NDB from a Helicopter (along with the V-force / FJ attack options for the weapon) which is not as suicidal as it sounds.
http://www.nuclear-weapons.info/vw.htm